The measurement of the stress field and its natural heterogeneity is a common challenge in the field of geophysics and underground engineering. This study presents findings from an extensive mini-frac campaign conducted at the Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Six dedicated stress measurement boreholes, spanning 500m along the Bedretto Tunnel, were utilized to prepare for meso-scale hydraulic stimulation experiments in a granitic rock volume. A detailed analysis of the induced hydraulic fracture trace morphology was carried out using acoustic televiewer logging data, with an interpretation based on the local geological context. The results show that the far field orientation of the maximum horizontal stress () is on average N112°E and the overburden stress can be assumed to be a principal stress direction. However, the en-echelon shape of the observed hydraulic fracture traces suggests that there is a slight inclination of the greatest principal stress away from the vertical stress direction. The mini-frac test intervals along boreholes inside the enlarged niche of the BedrettoLab indicate a counterclockwise rotated horizontal stress by up to 37° and potentially stronger inclination of the greatest principal stress from the vertical stress direction. The most likely causes of this stress field rotation, along with a considerably larger scatter in stress magnitudes, appear to be the larger laboratory niche dimensions compared to the tunnel and the presence of natural fractures that are preferentially oriented for slip in the far field stress state. Although the uncertainties and variability of the stress field around the BedrettoLab are not yet fully understood, our results are useful for strategic planning and design of meso-scale stimulation experiments in the rock volume. Furthermore, our detailed analysis procedure contributes to advancing the understanding of mini-frac tests in similar geological settings.

中文翻译：

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在 BedrettoLab 中约束应力场及其变化：精细的水力裂缝痕迹分析


应力场及其自然非均质性的测量是地球物理和地下工程领域的常见挑战。本研究介绍了在瑞士阿尔卑斯山贝德雷托地下实验室 (BedrettoLab) 进行的广泛小型压裂活动的结果。沿 Bedretto 隧道分布了 6 个专用应力测量钻孔，跨度 500 m，用于为花岗岩体中的细观水力刺激实验做准备。使用声波电视测井数据对诱发水力裂缝痕迹形态进行了详细分析，并根据当地地质背景进行了解释。结果表明，最大水平应力()的远场方向平均为N112°E，可以假定上覆应力为主应力方向。然而，观察到的水力裂缝痕迹的阶梯形状表明最大主应力远离垂直应力方向有轻微的倾斜。 BedrettoLab 扩大的壁龛内沿钻孔的小型压裂测试间隔表明，水平应力逆时针旋转达 37°，并且最大主应力与垂直应力方向的倾斜可能更大。这种应力场旋转的最可能原因以及应力大小的相当大的分散，似乎是与隧道相比更大的实验室生态位尺寸以及存在天然裂缝，这些裂缝在远场应力状态下优先定向滑动。 尽管 BedrettoLab 周围应力场的不确定性和可变性尚未完全了解，但我们的结果对于岩石体积中细观尺度刺激实验的战略规划和设计很有用。此外，我们详细的分析程序有助于增进对类似地质环境中小型压裂测试的理解。